1. Field of the Invention
The present invention relates to inkjet printing apparatuses and inkjet printing methods wherein an image is formed by scanning printing means across a print medium, the printing means ejecting a plurality types of ink thereupon.
2. Description of the Related Art
Inkjet printing apparatuses have a variety of advantages, such as being able to print at high densities and speeds, and being a printing method that is quiet and whose running costs are low. Inkjet printing apparatuses are being commercialized in a variety of configurations, being used as the output device for various apparatuses and in portable printers. Particularly in recent years, a great number of printing apparatuses that form a color image using a plurality of colors of ink have been provided.
An inkjet printing apparatus is typically provided with printing means (a print head) that ejects inks according to a printing signal, a carriage that houses this print head and an ink tank, conveying means that conveys the print medium, and control means that controls these components. In serial scan type inkjet printing apparatuses, a printing operation in which the carriage serially scans in a main print scan and a conveying operation in which the print medium is conveyed in a sub-scan direction that intersects the main print scan direction, are repeated alternately. Herewith, an image is gradually formed. Ink tanks for four or more colors are mounted on the carriage, and by printing these inks in single colors or in combinations of colors on the print medium, a full color image can be formed.
Meanwhile, it is known that in inkjet printing methods the order in which these plurality of inks are applied to the print medium can have a variety of effects on the print image. For example, Japanese Patent Laid-Open No. 2002-248798 discloses how, depending on the order in which ink is applied to the print medium, the chromaticity (i.e., the colors in the image) changes in image. This literature states that, with respect to inkjet paper, the initially applied inks exhibit stronger color.
In addition, Japanese Patent Laid-Open No. 2005-81754 discloses a technology that improves scratch resistance of an image by applying an additional a coating liquid after forming an image using coloring inks. Scratch resistance herein refers to an image's resistance against rubbing or scraping the print material with a fingernail or cloth. Since this type of a coating liquid shows its effectiveness by being applied to the print medium after image formation, its effectiveness is reduced if applied before image formation.
As described above, the quality of print material can be further improved for inkjet printing apparatuses by deliberately adjusting the application order of the inks. For this reason, in order to control the application order of the inks with respect to the print medium, the arrangement of the nozzle rows that eject each color/type of ink becomes an important element.
Typically, print head configurations in serial type color inkjet printing apparatuses can be divided into two broad types. The first is a vertically-arranged configuration wherein the nozzle row for each color is arranged in the sub-scan direction on the print head. The second is a horizontally-arranged configuration wherein the nozzle row for each color is arranged horizontally in the main scan direction. Hereinafter, these configurations will be described in turn.
FIG. 1 is a schematic diagram for illustrating the vertically-arranged print head configuration. Herein, a yellow ink nozzle row 15Y, a magenta ink nozzle row 15M, a cyan ink nozzle row 15C, and a black ink nozzle row 15K are disposed on a print head 151 in a single line in the sub-scan direction, such that the nozzle rows are mutually non-overlapping. In a vertically-arranged configuration such as this, in one main print scan of the print head, each color of ink is applied to a different region of the print medium. As a result, the order in which ink is applied to the print medium is black cyan magenta yellow, regardless of whether the print head 151 performs a main print scan in either the forward direction or the backward direction. For example, for a blue image, being expressed by a mixture of cyan and magenta, ink is always applied in the order cyan→magenta. For a print head configuration such as this, an image can be formed with the target ink application order without conducting any particularly difficult control, even when using a coating liquid like that described above. The nozzle row for ejecting the coating liquid need only to be positioned farthest downstream in the sub-scan direction.
However, for the print head of vertically-arranged configuration such as this, although the application order of ink with respect to the print medium can be fixed, it is difficult to change this order according to conditions. In addition, since the nozzle rows for each color are arranged in a single line in the sub-scan direction, there is a tendency for the print head to become lengthened in the sub-scan direction. Lengthening of the print head leads to enlargement of the entire apparatus, makes the mechanism that presses on the print medium more complex, and leads to higher costs for not only the print head itself, but also the apparatus as a whole.
FIG. 2 is a schematic diagram for describing the horizontally-arranged configuration of the print head. In this case, a yellow ink nozzle row 17Y, a magenta ink nozzle row 17M, a cyan ink nozzle row 17C, and a black ink nozzle row 17K are disposed on the print head parallel to the main scan direction. In a horizontally-arranged configuration such as this, there is not a tendency for the print head to become lengthened compared to the vertically-arranged configuration. Thus a printing apparatus that is comparatively compact and low cost can be realized.
However, in the case of the print head of horizontally-arranged configuration, each color of ink is applied to the same region of the print medium during one main print scan of the print head. Consequently, for scans in the forward direction, ink is applied in the order yellow→magenta→cyan→black, but for scans in the backward direction, this order is reversed. As described in the foregoing, in ink jet printing, the coloration of the reproduced image changes according to the order in which inks are applied to the print medium. Consequently, this reversal of the ink application order that occurs in each print scan is a factor that degrades image quality. For example, for blue images, which are reproduced by a mixture of cyan and magenta, alternating bands appear in which ink is applied in the order cyan magenta to form one band, and magenta cyan to form the other band. The result is clearly visible as color banding.
In order to cope with this problem, inkjet printing apparatuses typically implement a printing method known as multi-pass printing. In multi-pass printing, image data which can be printed by one main print scan is thinned out according to a mask pattern prepared in advance. The image data is gradually printed by a plurality of main print scans.
FIG. 3 is a schematic diagram for simply describing multi-pass printing. In this case, FIG. 3 illustrates the state wherein an image is printed on a print medium 52 using a print head 51 via 4-pass multi-pass printing. Between each main print scan, the print medium 52 is conveyed in the sub-scan direction by a quantity d equivalent to ¼ of the print width of the print head. In a print method such as this, an image of a unit region of the print medium 52 is completed by four main print scans corresponding to four regions 1-4 of the print head. As a result, the plurality of dot arranged on the print medium in the main scan direction are printed by four different nozzles. This alleviates adverse affect of nozzle variations, and the overall image becomes smoother. In addition, even when conducting bi-directional printing, ink will be applied to all of the image regions of the print medium 52 by both forward scans and backward scans. Thus the order in which ink is applied to the print medium will not differ depending on the band, and to some degree color banding appears in the overall image is curtailed.
FIG. 4 illustrates an example of mask patterns used when performing 4-pass multi-pass printing as in FIG. 3. In this case, a single color nozzle row 56 and its corresponding mask patterns 57a-57d are shown for the sake of simplicity. Nozzles in the nozzle row are divided into four regions. The nozzles contained in the respective regions print dots according to the mask patterns 57a-57d corresponding to each region. The individual mask patterns 57a-57d are comprised of a plurality of pixel areas, each pixel area determining whether the printing of a dot is permitted or not permitted. In FIG. 4, the black areas are the pixels where the printing of a dot is permitted, and the white areas are the pixels where the printing of a dot is not permitted. The four types of mask patterns 57a-57d exist in a complementary relationship, and by taking the logical product of these mask patterns and the image data for each print scan, the actual printed dots for each main print scan are determined. For the sake of simplicity, mask patterns having a 4-pixel×3-pixel area are shown herein, but practical mask patterns have much larger areas in both the main scan direction and the sub-scan direction.
If a multi-pass printing method such as this is implemented, the printing ratio for each region of the mask patterns can be made different in every color, even when using a print head of horizontally-arranged configuration. It is also possible to control to a certain degree the order in which ink is applied to the print medium, similar to the print head of the vertically-arranged configuration.
FIG. 5 illustrates an example of mask patterns devised such that, from among four colors, only yellow ink, for example, is to be applied to the print medium after other inks as much as possible. A nozzle row 61 illustrates the nozzle rows for cyan, magenta, or black, all of which print an image according to mask patterns 63a-63d. On the other hand, a yellow nozzle row 62 prints an image according to mask patterns 64a-64d. By preparing in advance such mask patterns, cyan, magenta, and black are used to print an image in four main print scans, each scan printing 25%. On the fourth pass, 100% of the yellow printing is conducted at the same time. As a result, there is a higher probability that the yellow ink will be applied after the application of the other inks.
However, for cases wherein mask patterns such as those illustrated in FIG. 5 are implemented, yellow ink can only be printed using the nozzles contained in region 4. In other words, even in cases where color banding is not a concern, such as when printing yellow as a single color, the nozzles for regions 1-3 are not used. If such a bias in the usage frequency among the plurality of nozzles exists in the nozzle row, the alleviation adverse affect of nozzle variability and the smoothing of the overall image, i.e., the original merits of multi-pass printing, are easily impaired. In addition, nozzle life depends on the number of nozzle ejections, and the print head may be declared unusable if even one nozzle exhibits ejection trouble. In other words, causing bias in usage frequency within the nozzle row leads to the shortening of the life of the print head itself.
In configurations of the conventional art like that described above, devices are controlled such that specific ink (yellow ink) is always applied with a scan subsequent to that of non-specific ink (cyan, magenta and black ink), regardless of the overlapping conditions of the specific ink and the non-specific ink. Thus, the above-described bias has been larger than necessary.